1. Field of the Invention
This invention relates to a radio communication system for make communication between a base station and a mobile communication terminal via a radio channel. More specifically, the present invention relates to a radio communication and its mobile communication terminal for adaptively controlling a transmission parameter in response to its channel quality for each mobile communication terminal.
2. Description of the Related Art
In a radio communication system, for example, a high-rate packet data (HRPD) system of CDMA 2000 is known as a scheme for adaptively controlling a transmission parameter in response to its channel quality. This system is used for an exclusive radio interface for a packet exchange network, and in a downstream radio channel from a base station to a mobile communication terminal, this system transmits packet data with time-division multiplexing by assigning a time slot to a plurality of mobile communication terminals, respectively.
The radio communication system with the HRPD system adopted therein adaptively controls average throughput to a specific mobile communication terminal in response to a channel quality of each radio channel. This control is achieved, for example, as follows. That is, the mobile communication terminal measures the channel quality used between the communication terminal and the base station. Then, the communication terminal decides a transmission format necessary for maintaining an error rate constant on the basis of the measurement result and the requests the decided transmission format to the base station. The base station transmits the packet data to the communication terminal by using the transmission format.
By the way, the downstream format considered in the HRPD system is decided by a combination of a plurality of transmission parameters such as a data rate, a packet size, a modulation system and an encoding rate. Accordingly, it is needed to select a combination of a high data rate and a high-efficiency modulation system in order to obtain high throughput. However, in a radio communication system with the general HRPD system adapted therein, a transmission format capable of achieving the high throughput is selected when a signal interface noise ratio (SINR) of the ratio channel measured by the communication terminal is good. In contrast, when the SINR is decreased, a transmission format excellent in interference resistant characteristics is selected so as to maintain the error rate to the channel quality at a prescribed value or more. Therefore, the average throughput is decreased under an environment with a poor channel quality as such case that the mobile communication terminal is located, for example, in the vicinity of a cell boundary between base stations.
Consequently, for example, as described in Jpn. Pat. Appln. KOKAI Publication No. 2002-58063, a technique is proposed conventionally to increase transmission power of a downstream radio channel for the communication terminal located in the vicinity of the cell boundary in a time zone preset so as not to be overlapped with an adjacent base station. According to this technique, it becomes possible to improve a ratio of carrier wave power/interference wave (C/I) of the communication terminal located in the vicinity of the cell boundary and enhance its throughput.
Although, the conventional technique described above is effective in the case that the decrease in the throughput of the communication terminal results from the location of the communication terminal in an area with a low SINR, however, the effect cannot be expected if the decrease in the throughput is resulted from a factor other than the decrease of the SINR, for example, a factor of concentration of traffic.